Hydroquinone-quinone system

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

The role of ubiquinone (coenzyme Q, 4) in transferring reducing equivalents in the respiratory chain is discussed on p. 140. During reduction, the quinone is converted into the hydroquinone (ubiquinol). The isoprenoid side chain of ubiquinone can have various lengths. It holds the molecule in the membrane, where it is freely mobile. Similar coenzymes are also found in photosynthesis (plastoquinone see p. 132). Vitamins E and K (see p. 52) also belong to the quinone/hydroquinone systems. 

There are several carbon compounds that exhibit reversible behavior. We refer to the quinone-hydroquinone system, which is used in pH-sensitive electrodes.) The solubility of CO2 in 100 g water at 1 bar and 20 °C is 0.172 g. 

A similar mechanism could operate in the reduction of oxygen on chelate catalysts, as in the organic cathodes with air regeneration described by Alt, Binder, Kohling and Sandstede 13-40>. These cathodes contain a reversible insoluble quinone/hydroquinone system. The quinone, which is electrochemically reducible, can be obtained either by electrochemical oxidation or by purely chemical oxidation with H2O2 or oxygen (air). A cathodic current is observed in these systems only at potentials below the redox potential, and unusually hard current/ voltage characteristic curves are obtained. 

Note that the anodic peak due to the oxidation of leucoadrenochrome to adrenochrome near 0 V is not seen until the second positive-going potential sweep is made. The voltage separation between the anodic and cathodic peaks for the oxidation of adrenaline (peak B, Fig. 21.4, bottom) and the reduction of adrenalinequinone (peak C) is large when compared to most chemically reversible redox couples. However, this behavior is typical of many quinone-hydroquinone systems on a carbon paste surface at intermediate values of pH. 

Apart from these systems, which involve a metal on which is adsorbed a modifier (see above), there is another kind of experiment, although one from which data are as yet less available. One can make up a surface of a metal covered with a biolipid membrane (90% lipid and 10% proteins). There is evidence that some of the proteins in these ensembles are themselves the origin of electrons that can exchange with small redox molecules (e.g., the quinone-hydroquinone system) in solution. Such evidence (though scarce) is significant, for there are no metal underlayers in real biological systems, yet interfacial electron transfer seems to be common there. 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

Fig. 25. Dependence of faradaic resistance measured at the equilibrium redox potential on the polycrystalline film resistivity for (1) Fe(CN)63, 4 and (2) quinone/hydroquinone systems. Reprinted from [110], Copyright (1997), with permission from Elsevier Science.

Redox systems other than the H2/H+ couple can be used to monitor the potential of the parent metal particles. For example, the quinone-hydroquinone system can be used to keep the electrochemical potential between 0.5 and 0.0 V/NHE by varying the pH from 0 to 7 for solutions of equal concentrations of quinone and of hydroquinonc [57], UPD clearly opens up a vast... 

Quinone-Hydroquinone Systems.—In the brief treatment of the quinone-hydroquinone system on page 270 no allowance was made for the possibility of the hydroquinone ionizing as an acid actually such ionization occurs in alkaline solutions and has an important effect on the oxidation-reduction potential of the system. Hydroquinone, or any of its substituted derivatives, can function as a dibasic acid. It ionizes in two stages, viz.,... 

The reversible quinone-hydroquinone system also behaves in a somewhat unusual manner at a platinized platinum electrode there is little polarization other than that due to concentration changes at the electrode, both for oxidation and reduction. With other electrode materials, however, there is marked polarization, especially as the c.d. is increased at sufficiently high currents diffusion becomes rate determining, but at lower values the nature of the slow process is not at all clearly understood. The polarization appears to be influenced in an unexpected and complex manner by the hydrogen ion concentration of the electrolyte. ... 

Other redox systems of importance in biochemistry include the NADH/NAD system, the flavins, the pyruvate/lactate system, the oxalacetate/malate system, and the quinone/hydroquinone system. 

The quinone-hydroquinone system can be involved in redox mechanisms. So, in addition to acid-base catalysis, carbon materials can promote oxidations, such as the oxidative dehydrogenation of hydrocarbons, which we discuss in detail below. 

Taking advantage of the high reversibility of the quinone - hydroquinone system, a stronger adsorption of the compound, and hereby a better sensitivity, should be obtained if the preconcentration step is performed at a potential more negative than the reduction one, followed by a positive going potential scan. 

Some of the recently reported quinone-based redox polymers (Manecke, 1974) have the structural units represented in Fig. 12-1. In these polymeric quinone s, an attempt has been made to increase the hydrogen acceptor property of the quinone group by introducing electron-withdrawing substituents. Such substituents are known to increase the redox potential of the quinone-hydroquinone system (cf. 2,3-dichloro-... 

Quinone Catalysis. We have already shown that electrons can easily be removed from hydroquinone and that the loss of electrons is equivalent to an oxidation to the quinone. One such quinone-hydroquinone system is in all likelihood interposed as a hydrogen carrier in the respiratory chain between the flavoproteins and the... 

---